Lake property owners’ worst fear is coming true: The zebra mussel population in Lake Minnetonka is spreading rapidly, according to researchers studying the distribution and density of the invasive species.

Zebra mussels were first discovered in Wayzata Bay in 2010. The Minnehaha Creek Watershed is in the middle of a three-year study to figure out where and how the zebra mussels are spreading and to determine how rapidly they are multiplying.

The watershed today released results from June through September showing zebra mussels have spread to nearly all of the lake’s eastern bays and are moving to western areas of the lake.

The study is unique in Minnesota in that it’s looking at both the distribution and density over an extended period of time, watershed spokeswoman Telly Mamayek said.

She said researchers are receiving reports from the public about zebra mussels attached to boats and docks being pulled out of the water for the season. Zebra mussels can also be a nuisance for swimmers, as the mussels’ shells can cut feet. The mussels also alter the food chain in lakes.

The second year of the study will begin next spring.

The maps below from the Minnehaha Creek Watershed District show where zebra mussels were detected in December 2010 vs. December 2011. The red marks indicate spots where there were zebra mussels, whereas the blue marks indicate where they were absent.